1. Field of the Invention
The invention relates to a process and an apparatus for guiding a coated material s trip in an initial drying zone, without requiring mechanical contact, by utilizing a heated gas.
2. Description of the Related Art
It is currently the state of the art to perform a surface treatment in floating film dryers for film or metal strips utilizing the aid of a supporting-air nozzle system which permits the material to be guided in the dryer without requiring mechanical contact. (journal "gas warm international", Volume 24 (1975), No. 12, pp. 527-531). In such dryers, the dryer air which is enriched with solvent is extracted directly in the nozzle zones in order to eliminate an undesired transversal flow. This produces so-called nozzle dryers or impact-jet dryers, in which a particular disadvantage is the impact of the point-like flow of individual nozzles, which tends, both in the case of laminar flow form and in the case of turbulent flow form, to cause flow-physical instabilities. The flow-physical instabilities, particularly in the case of low-viscosity liquid films, inevitably results in irreversible drying structures.
To avoid impact point-like flows in the beginning of the dryer apparatus, PCT Application WO 82/03450 discloses that the dryer air is directed out of an antespace, via suitable inlet openings and flow deflectors, and into a stabilized intermediate space. From the intermediate space, part of the dryer air passes via a porous filter element, arranged in the direct vicinity of the liquid jet, to the web to be dried. The operating principle of such drying is based on the fact that between the porous protective shield and the liquid film to be dried, there is formed a stabilized, yet highly solvent-enriched, weak air flow, which is constantly renewed by exchange with the residual air flowing transversely over the porous medium. Consequently, by virtue of the relatively short overall dryer length, a predrying of the liquid film with a reduced tendency for exhibiting mottling effects is accomplished.
This type of drying is predominantly characterized by the diffusion of the solvent vapor/air mixture through the porous protective shield. A complete drying out of the liquid film is only possible by having very great dryer lengths or by utilizing auxiliary dryers further downstream, because of the virtually complete lack of convective removal within the space that exists between strip and protective shield.
In the drying of products in web forms which have large surface areas on which liquid layers are applied, different drying processes and drying apparatuses have been used. Typical drying products are, for example, metal or plastic strips on which liquid layers are applied. As a rule, these liquid layers are composed of vaporizable components, which are removed from the liquid film during the drying process, and of non-vaporizable components, which remain on the base material after drying. In general, the webs which are to be dried are initially passed through an initial drying zone and subsequently through an actual drying zone. The coating gives the surfaces of the base materials special properties, which are not present in the form in which they are desired for later use until after the drying process. AS an example of this, mention can be made of the coating of metal strips, in particular aluminum strips, with light-sensitive layers, which are made into printing plates. The coating of metal strips or plastic sheets with a solvent-containing wet film, referred to hereinafter as a liquid film, and its subsequent drying, constitute an operation which requires special installations in order to ensure the desired product quality of the layers. What is essential in this case are the process steps of the initial drying and the final film drying as the final process measures of the coating.
The uniform drying of the blasting-sensitive coatings on the base material in dryers in which a dryer air flow runs parallel to the coated strip, is primarily disturbed by mechanical guide elements, such as rolls, for the base material and the coated metal strip. Very good drying results, with regard to the layer cosmetics, are obtained utilizing corresponding air conduction on the layer side that is to be dried if the material strip is guided in close proximity to the bottom of the dryer. If the material strip is passed, for example, over a level dryer bottom, which is composed for example of aluminum, there is the risk that so-called longitudinal scratches will occur on the underside of the material strip which can damage the coated material strip or even render it unusable.
If the bottom of the dryer is lined with a soft material,, such as for example a woven fabric, felt or a nonwoven fabric, the aforementioned longitudinal scratches are avoided. However, due to the different widths of the material strips, these lining materials are cut into. When the lining materials are cut into, particles of the lining material are formed which become deposited on the layer to be dried. Consequently, the layer to be dried becomes soiled and the lining materials become scored, which prevents a uniform contact with the guiding surface.
If the bottom of the dryer is lined with a material which has a substantially lower friction value than the material strip and which is temperature-resistant, such as for example polytetrafluoroethylene, this material will not remain mechanically resistant with respect to the edges of the material strip over an extended period.